1. Field of the Invention
The present invention relates to plasma spraying systems, and more particularly to arrangements for introducing powder in controlled fashion into a gas stream in a plasma spraying environment.
2. History of the Prior Art
Plasma spraying systems in which powder is selectively introduced into a flowing gas stream are well known. An example of such systems if provided by U.S. Pat. No. 4,328,257 of Muehlberger et al. which issued May 4, 1982 and which is commonly assigned with the present application. The Muehlberger et al. patent describes an arrangement in which a powder containing gas stream is introduced into a chamber at high temperatures and supersonic speeds to effect spraying of a work piece.
In plasma spraying systems of the type described in the Muehlberger et al. patent, the powder may be introduced into the flowing gas stream using various different designs of powder feeders. In a typical prior art powder feeder, the powder is loaded into a cylindrical cannister mounted at a 45.degree. angle and having a slotted wheel at the lower end thereof. The wheel rotates so as to fill the slots with powder, following which the slots are moved into the path of a gas stream so that the powder within the slots is entrained into the stream. Powder feeders of this type have been found to have a number of shortcomings including principally nonuniformity in the supply of the powder.
This has led to the development of an improved powder feeder which is described in a co-pending application of Muehlberger et al., POWDER FEEDER, Ser. No. 387,356, filed June 11, 1982 and commonly assigned with the present application. The powder feeder described in the Muehlberger et al. application locates the powder cannister in an upright, generally vertical position and utilizes a stir spindle and a feed impeller mounted on a rotatable first drive shaft at the bottom of the cannister to agitate and mix the powder and then dispense the powder in controlled amounts from an aperture in the bottom of the cannister as the first drive shaft is driven by a motor. The controlled amounts of powder dispensed through the aperture in the bottom of the cannister are directed by a conduit through an aperture in the upper end of a housing and into a plurality of slots circumferentially formed about the outer periphery of a feed wheel mounted within the housing to be rotatably driven by a second drive shaft which is also coupled to be driven by the motor. The slots which are uniform in size have bottom surfaces and opposite side surfaces formed by opposite vanes extending upwardly from the upper surface of the feed wheel between a first hollow tube mounted in a fixed location relative to the feed wheel so as to direct a gas flow through the slots and an opposite second hollow tube disposed to receive the gas flow and the powder loaded into the slots. The constant action of the stir spindle, the feed impeller and the feed wheel provides a relatively constant, uniform supply of the powder to the gas stream, even in the face of varying operating conditions.
In the powder feeder described in the Muehlberger et al. application and in other types of powder feeders such as the one previously described, the gas flow is directed through a hose, tube or other conduit into the powder feeder and then through an area where controlled amounts of powder are disposed to another conduit which directs the resulting mixture of gas and powder into the plasma stream. When spraying is to begin, the source of pressurized gas is turned on to begin flow of the gas through the conduits and the powder feeder. A period of several seconds or longer is typically required in order to build up pressure within the powder feeder to a level at which spraying can commence and powder flow is constant. In the meantime, both time and powder are wasted. When spraying is terminated it is usually desirable or necessary to rid the system of excess powder in preparation for the next spraying operation. This operation which involves blowing residual powder out of the conduits and the powder feeding areas within the powder feeder can require as much as ten seconds or longer to perform and is also wasteful of both time and powder. Certain types of plasma spraying operations require that the powder be supplied during a succession of intervals of several seconds each. Present powder feed control systems typically require substantial delays between the short spraying intervals while the system is cleared of residual powder and then spraying pressure is again built back up after each signal to again commence spraying. The practical result is that such spraying operations require an inordinate amount of time as well as being wasteful of powder.
Accordingly, it would be desirable to be able to start and stop the feeding of powder with little or no time delays and at the same time in a manner which minimizes wastage of the powder. It would also be desirable to provide an arrangement for ridding the powder cannister of excess, entrapped air after it is loaded with powder so that a desired pressure within the powder feeder for optimum powder feeding can be quickly established and maintained. It would furthermore be desirable to provide an improved valve for use in powder feed control systems as well as in other applications where it is desired to quickly and effectively shut off a flowing powder-gas mixture.